A heat pipe is essentially a passive heat transfer device with an extremely high effective thermal conductivity. A two-phase heat transfer mechanism results in heat transfer capabilities from one hundred to several thousand times that of an equivalent piece of copper. Heat pipes are sealed vacuum vessels that are partially filled with a fluid, typically water in electronic cooling applications, which serves as the heat transfer medium. The heat pipe envelope is typically made of cylindrical copper tubing, although rectangular cross sections and other materials are available. The wall of the envelope is lined with a wick structure, which generates the capillary force that pulls the condensate from the condenser section of the heat pipe back to the evaporator section. Since the heat pipe is evacuated and then charged with the working fluid prior to being sealed, the internal pressure is set by the vapor pressure of the working fluid. As heat is applied to a portion of the surface of the heat pipe, the working fluid is vaporized. The vapor at the evaporator section is at a slightly higher temperature and pressure than other areas and creates a pressure gradient that forces the vapor to flow to the cooler regions of the heat pipe. As the vapor condenses on the heat pipe walls, the latent heat of vaporization is transferred to the condenser. The capillary wick then transports the condensate back to the evaporator section. This is a closed loop process that continues as long as the heat is applied.
The orientation and layout of a heat pipe design are important. When the design allows, the heat source should be located below or at the same elevation as the cooling section for best performance. This orientation allows gravity to aid the capillary action, and results in a greater heat carrying capability. If this orientation is unacceptable, then a capillary wick structure such as sintered powder will be necessary. Additionally, heat pipes have the ability to adhere to the physical constraints of the system, and can be bent around obstructions.
There is a recurring need for heat pipes having low mass. There has been an extended effort to devise a method for using aluminum as the envelope and wick material. Much of this effort has been to use water as the preferred working fluid. Previous efforts have been focused on taking advantage of the fact that aluminum oxide is compatible with water, even though aluminum metal is not compatible. The programs have not been successful because of the large difference in thermal expansion between aluminum and its oxide. The resulting stresses cause the oxide layer to crack, often on the first thermal cycle, thereby allowing the water and aluminum to come into contact, resulting in hydrogen generation and heat pipe failure.
The present invention takes advantage of the stabilizing effects of the “getter” type materials, such as zirconium when added to light metals such as magnesium or aluminum. The addition of zirconium to the magnesium provides a more stable oxide and/or nitride, and provides a water-compatible surface. The fact that this alloy is also lighter than aluminum is an added benefit. The reduced thermal stresses which result with this alloy most likely allow the oxide/nitride to maintain its integrity.
Most commercially available magnesium alloys have significant amounts of aluminum, rare earths, and/or zinc as constituents. None of these materials are readily compatible with water. Therefore, an additional objective of the present invention is to specify a water-compatible alloy of magnesium which does not have these non-compatible constituents.